Petra and the others initially have the upper hand since they blind the Female Titan with the vertical maneuvering gear's gas and cripple both her arms, making them useless

But when Petra and Eld Jinn move in for the kill, the Titan suddenly regenerates a single eye, demonstrating a previously-unknown ability to focus healing into fewer body parts to speed up the regenerative process as a defensive tactic, and bites Eld in half, killing him

"Just one eye?! You concentrated on the one eye so it'd heal faster?! How can you...do that?!"

Petra was paralyzed in shock from what she just witnessed. If she doesn't react in time, the female titan would kill her like it had just killed Eld